Cage-type intervertebral implant

ABSTRACT

The invention relates to a cage-type intervertebral implant that is made up of a dished side wall ( 1 ), a cambered side wall ( 2 ), a front part ( 3 ), a rear part ( 4 ) and at least one intermediate wall ( 5, 6 ), thus comprising at least two cavities ( 7, 8, 9 ). An upper and a lower cage surface ( 10, 11 ) include a first lordosis angle (α 1 ) in the direction front part-rear part and a second lordosis angle (α 2 ) perpendicular thereto, said cage surfaces ( 10, 11 ) intersecting outside the cage. The cage structure is characterized by a doubler-wedge geometry (double-wedge-shaped cage) that is defined by the two lordosis angles (α 1 ) and (α 2 ) and that advantageously adapts itself to the anatomical conditions in the intervertebral area. The cage is further characterized by a high moment of tilt that effectively counteracts a tipping of the cage. The method used for producing the cage structure is essentially characterized by working the cage material by means of a high-pressure water jet, said cold-cutting technique having proved to be the most economical.

[0001] The invention relates to a cage-type intervertebral implant, and also to a method for its production, according to patent claims 1 and 23 respectively.

[0002] It relates to a vertebral column implant and its method of production, the implant being used for insertion between two vertebrae of the vertebral column. It serves as a means of fusion (arthrodesis) of the two vertebrae, and conse-quently the original intervertebral disk height is again reached by means of it, and also the nerve root foramen returns to its original size.

[0003] The individual vertebrae of the vertebral column have a vertebral body, a vertebral arch, a spinous process, two transverse processes, and two upper and two lower articular processes. The vertebrae are connected to the abutting intervertebral disks (disci intervertebralis) and give rise to the vertebral body (corpus vertebrae). The intervertebral disk consists of liquid-rich fibrous cartilage, and connects the individual vertebral bodies together. The size of the intervertebral disks increases from top to bottom, corresponding to the loads arising in the human body. The intervertebral disks serve as elastic buffers, and resiliently damp impacts.

[0004] It is known that the intervertebral disks can become displaced, or that the inner gelatinous core (nucleus pulposus) can emerge through cracks in the cartilaginous outer ring (annulus fibrosus), which is similar to connective tissue. The intervertebral disk can then partially enter the intervertebral foramina (foramina intervertebralia) or into the spinal canal. Furthermore, this prolapse can be dorsal, medial, or lateral. Such prolapses most frequently occur at the L4-L5-S1 and C6-C7 vertebrae. If such prolapses are not treated, irreversible pressure damage of nerve roots, foramina, or transverse lesions, result. If physiotherapy according to the symptoms, e.g., remedial exercises or massage, show no promise of success, the intervertebral disk (discus intervertebralis) has to be operatively removed. There now exists the possibility of implantation of such an implant (cage), by means of which an arthrodesis between the two vertebral bodies can take place.

[0005] An intervertebral implant is known from EP 0916323-A1 which has a bean-shaped structure and can be inserted between two vertebrae. The implant has a wedge shape, conferred by a different height of the two longitudinal sidewalls. The walls surrounding the implant are provided with rows of holes in order to promote the ingrowth of bone tissue.

[0006] It is disadvantageous that the implant has a wedge shape in only one direction, and is expensive to manufac-ture because of the many laterally formed holes.

[0007] Furthermore, cage structures are known under the designation “Brantigan cage”, which have many teeth on their cage surfaces in order to prevent an undesired displacement of the cage. Made of polyether ether ketone (PEEK), as so-called PEEK moldings, they have inadequate strength, which can lead to breakage of the cage structure under load. A single thread is provided to receive instruments, resulting in unsatisfactory instrument manipulation.

[0008] A relatively small tilting moment is conferred by the cuboidal geometry, with disadvanta-geous effects.

[0009] The invention has as its object to provide a cage-type intervertebral implant which has a double wedge-shaped geometry, depending on two lordosis angles, and which ensures an improved instrument manipulation.

[0010] A further object of the invention consists of the production of such an implant.

[0011] According to the invention, this object is attained with an implant according to the wording of patent claim 1 and by a method of production of the same according to the wording of patent claim 23.

[0012] The invention is described hereinbelow using the accompanying draw-ings.

[0013]FIG. 1 shows a perspective view of a cage

[0014]FIG. 2 shows a plan view of the cage according to FIG. 1

[0015] FIGS. 3A-3D show sectional views of FIG. 2

[0016] FIGS. 4A-4C show sectional views of FIG. 2

[0017]FIG. 5 is a sectional view of FIG. 2 along the developed radius line a-a@

[0018]FIG. 6A-6B show side views of the rear portion with differently arranged guide ele-ments

[0019]FIG. 7 is a plan view of an embodiment example of a cage with a partition and oblique rear portion with openings.

[0020]FIG. 1 shows a cage 100 in a perspective view, consisting of a concave curved sidewall 1, a convex curved sidewall 2, a front portion 3 and a rear portion 4. The sidewalls 1 and 2 are connected together by partitions 5 and 6, so that the interior of the cage is divided into cavities 7, 8 and 9. A first, inner, radius of curvature R1 is then allocated to the internal sidewall 1, and a second, outer, radius of curvature R2 is allocated to the outer sidewall 2.

[0021] The sidewalls 1, 2, the partitions 5, 6, the front portion 3 and the rear portion 4 have upper, or lower, respective boundaries which define an upper or a lower cage surface.

[0022] The concave curved sidewall 1 has rounded openings 13, 14 and 15, which are placed about in the middle of the cavities 7, 8 and 9, and are conducive to the formation of bone substance. The convex curved sidewall 2 can likewise have such openings (not shown).

[0023] The cage surfaces 10, 11 have, in the region of the front portion 3, the rear portion 4 and the partitions 5, 6, tabular raised portions 24, 25, 26, 27 which run substantially parallel to the cage surfaces and whose properties are described hereinafter.

[0024] The front portion 3 is rounded, and connects the sidewalls 1, 2 of the cage by means of an equal wall thickness. On the front side it has bevels 23, 23@ which facilitate the introduction and positioning of the cage in the interver-tebral region.

[0025] The rear portion 4 is of a rectangular constitution, and connects the sidewalls 1, 2 of the cage, likewise by means of an equal wall thickness. It has a bore 20 on the rear side which is provided with an internal thread and is provided for instrument attachment. Guide elements 21 and 22 are arranged on respective sides of the bore 20, and are constituted here as, e.g., ribs, but can also consist of openings in the form of a half cylinder. The guide elements serve to guide instrument introduction, and prevent any improper rotary movement of the cage when the instrument is released. As soon as the cage is situated in its final position between the two vertebrae, which among other things is the case when the axis of the instrument is perpendicular to the dorsal plane of the patient, the instrument can be detached from the cage. It has become apparent that this possibility of control has turned out to be very helpful and useful.

[0026] The transition of the guide elements, or ribs, 21, 22 to the surface of the rear portion 4 is rounded off on both sides of the ribs, in order to avoid possible notch effects, which is of importance in embodiments in plastic or composite materials. Holes 31, 32 or 33 are provided in the rear portion 4 and in the front portion 3, to receive a marker of a high density metal. Tantalum balls and/or pins are particularly suitable for this purpose. The pins are arranged in bores which are arranged either perpendicular or parallel to the bore 20. The position of the cage can thereby be observed and assessed during the operation by means of an image intensifier.

[0027]FIG. 2 shows a plan view of the cage 100 according to FIG. 1, with data on the position of the sections A-A@ through D-D@ and E-E@ through G-G@. The course of a radius line a-a@ which runs through the middle of the cage can likewise be seen.

[0028] FIGS. 3A-3D show sectional illustrations of the cage, with section posi-tions according to FIG. 2. FIG. 3A shows a section A-A@ through the rear portion 4. The hole 20 to receive an instrument, and the holes 31 and 32 for the markers, are visible. It can furthermore be recog-nized that the height at the outer side of the rear portion is greater than that at the inner side. Thus the upper surface of the rear portion no longer runs parallel to the lower surface, as portions of the cage surfaces 10 and 11. The two surfaces form a lordosis angle @2, which is 0.1-4°, but preferably 2°. This lordosis angle is shown on an exaggerated scale in FIG. 3A, to be seen more easily. It is shown by @2/2 at the lower side of the rear portion.

[0029] In the case that the height at the outer side of the rear portion is smaller than that at the inner side, there results an opposed slant of the cage, or a wedge shape formed by the cage surfaces 10 and 11, with the point of the wedge facing in the reverse direction If the lordosis angle @2 in the two described cases is identically zero, the cage surfaces 10 and 11 are then parallel, as a special case or bounding case, which of course represents a less preferred constitu-tion of the cage.

[0030] The raised portions 24, 24@ are affixed to the portions 10, 11 of the cage surfaces, and here are constituted parallel to the cage surfaces, although this by no means has to be obligatory.

[0031]FIGS. 3B and 3C show a section B-B@, or C-C@, through the partitions 5 or 6, with the raised portions 25, 25@, or 26, 26@, which are positioned on the portions of the cage surfaces 10, 11. These raised portions again run substantially parallel to the cage surfaces which likewise enable the lordosis angle @2 to be perceived.

[0032]FIG. 3D shows a section D-D@ through the front portion 3. The bore 33 for the marker can be seen The raised portions 27, 27′ can also be seen, which are affixed to the portions 10, 11 of the cage surfaces of the front portion 3. Again, these raised portions run substantially parallel to the cage surfaces, which likewise enable the lordosis angle @2 to be recognized.

[0033] The raised portions 24, 25, 26 and 27, which all project from the cage surfaces 10, 11, but are only 0.3-0.8 mm, serve for the “anchoring of the cage” after the successful operation, and help to prevent a migration of the cage.

[0034] FIGS. 4A-4C show sectional diagrams of the cage, with positions of the cross sections according to FIG. 2. There can be seen the sidewalls 1, 2; the upper and lower cage surfaces 10, 11; and the half lordosis angle @2, which is only shown in FIG. 4A on one side.

[0035]FIG. 5 shows a sectional diagram of FIG. 2 along the developed radius line a-a@. The hole 33 for the marker, the bevels 23, 23@, and the raised portions 27, 27@ can be seen in the front portion 3; the hole 20 and the raised portions 24, 24@ can be seen in the rear portion 4. The partitions 5, 6 respec-tively have the raised portions 25, 25@ or 26, 26@.

[0036] It can further be seen that the height of the front portion 3 is greater than that of the rear portion 4. Thus the cage surfaces 10 and 11 no longer run parallel. The two surfaces form a so-called lordosis angle @1, which is 2-8°, but preferably 3°, 5°, or 7°. This non-parallelism conditioned by the lordosis angle @1 is shown on an exaggerated scale in FIG. 5, to be more easily visible.

[0037] The cage structure can of course be modified within wide limits within the scope of this invention. Thus, for example, the number of the partitions 5, 6, or that of the cavities 7, 8, 9, is not limited to 2 or 3. Cage structures with one or more partitions are possible.

[0038]FIGS. 6A and 6B each show a side view of the rear portion with different guide elements 21 and 22 arranged around the hole 20.

[0039] In FIG. 6A, the guide elements are arranged about parallel to the cage surfaces 10, 11, while those in FIG. 6B have an angle of about 45° to the cage surfaces. However, this angle can assume an optional value from 0 to 90°. The guide elements 21, 22 in their turn of course do not necessarily have to be arranged parallel to one another; they can also have a V-shaped arrange-ment.

[0040] Possible materials are plastics, carbon fiber reinforced plastics and metals or metal alloys. Plastics such as polyether ether ketone (PEEK), polyether ketone ether ether ketone (PEKEEK) and polysulfone (PS) are preferably used, and particularly preferred as composite materials, carbon fiber reinforced composites of polyether ether ketone (CFK/PEEK) and polyether ketone ether ether ketone (PEKEEK), which are also known under the names of ULTRAPEK and OSTAPEK.

[0041] As metals or metal alloys, titanium and its alloys are preferably used, such as e.g., the titanium alloy Ti6-Al4-V according to ISO standard 5832-3.

[0042] The metallic cage can have a hydroxyapatite ceramic (HAK) coating or a tricalcium phosphate (TCP) coating, which advantageously affect the long-term properties of the implant.

[0043] The curved shape of the cage gives this an advantageous high tilting moment M, which effectively opposes an overturning of the cage. In comparison with the tilting moment of a cuboidal cage with equal middle cross section, equal length, and comparable cage structure, it has been found that the cage structures according to the invention exceed this by a factor of at least 1.30. For a cage according to FIG. 1, the factor is 1.58.

[0044] The advantages of the cage structure according to the invention result from the double-wedge geometry, which depends on the two lordosis angles @1 and @2, and which outstandingly matches the anatomy in the intervertebral region.

[0045] The designation “double wedge-shaped cage” or “DWS cage” is therefore used for such a cage.

[0046] The raised portions positioned on the cage surfaces effectively prevent a migration of the cage during the healing process after a successful operation.

[0047] Cage structures of the described kind are distinguished by high strength attained in spite of a small proportion of material. The formation of bone material is thereby strongly accelerated.

[0048] It has been found that this property can be described by a Cage Mass Index (CMI), which is defined according to Equation (1),

CMI=Volume of cage material/Volume of cage  (1)

[0049] namely as the ratio of the cage material volume to the total cage volume. The results are:

[0050] (a) for CFK/PEEK, CFK/PEKEK [sic], CFK/PS less than 0.25, preferably 0.22, and

[0051] (b) for titanium or Ti alloys, less than 0.20, preferably 0.17,

[0052] the variations being dependent on the cage sizes and giving only unimportant differences.

[0053] The process for the production of such a cage is described hereinafter. It is divided into of four process steps, as follows:

[0054] 1. Water Jet Cutting

[0055] In a first step, a blank of cage material is machined in a first direction by means of a high pressure water jet. This known and economical cold cutting process is as a rule operated with an abrasive addition at 3,000 bar (U. W. Hunziker-Jost, Swiss Precision Manufacturing Technique, p. 81-86, C. Hanser Verlag, Munich (1991)).

[0056] The blank is clamped so that the water jet is directed perpendicularly to the later cage surface. The contours of the sidewalls 1, 2, of the front portion 3, of the rear portion 4, of the at least one partition 5, 6, of the at least two cavities 7, 8, 9, and of the guide elements 21, 22 are cut with high precision The cut edges have little fraying. With material thicknesses of 10 mm, cutting speeds are attained of up to 100 mm/min for metals and up to 300 mm/min for composite materials.

[0057] 2. Milling

[0058] The cage blank cut from the blank in this manner is now clamped again in a second step, and in fact in a second direction, substantially perpendicular to the first direction, in which the cage blank is further machined with a miller. The surfaces milled are the cage faces 10, 11 corresponding to the lordosis angle @1, the wedge-shaped raised portions 24, 25, 26, 27 corresponding to the second lordosis angle @2, the beveled surfaces 23, 23@ of the front portion 3, the at least one opening of the sidewalls 1, and the first hole 20. Likewise in this step, the hole 20 is provided with an internal thread, which is intended to receive an instrument. Small-calibered milling inserts are used here on an automatic, computer numerically controlled milling machine.

[0059] If openings are likewise to be provided in the outer sidewall 2, the cage blank has to be newly clamped once more.

[0060] 3. Affix Markers

[0061] In a third step, the markers are mounted on the cage blank; later, during the operation and thereafter, they make it possible to assess the position of the cage by means of an image intensifier. Second holes 31, 32, 33 are installed for the markers in the rear portion and the markers in the front portion, and the markers are inserted into them as tantalum balls and/or pins.

[0062] 4. Finishing

[0063] The last operations, which are summarized as finishing, can take place in a fourth step, namely trovalization in order to de-burr or round off the partially sharp edges. Then follows the marking of the cage, which can take place by means of a laser marking device. The cage is next subjected to a cleaning process, which includes, for example, multi-step ultrasonic cleaning. The packaging of the cage likewise belongs in these finishing operations.

[0064] A more important process step is cutting with a water jet under high pressure. An advantageous cutting process was thereby selected, which has been found to be particularly effective.

[0065] The examples described hereinafter give an insight into the diversity of the cage design, and their enumeration is not to be considered as final in any way.

[0066]FIG. 7 shows in plan view, as an embodiment example, a cage with one partition and an oblique rear portion with openings.

[0067] Sidewalls 1, 2, front portion 3 with raised portion and bevel 23, partition 5 with raised portion, and the cavities 7, 9, correspond to the cage elements described in FIG. 1. The rear portion 4 with a portion raised with respect to the cage surface has here, however, a rhomboidal constitution. While the guide elements 21, 22 are constituted as recesses here, but are arranged on the rear portion surface 4@ as in FIG. 1, the direction of the hole for receiving an instrument is shown at the edge of the cavity 7 by the position of the axis 30 for the hole. The hole is provided with an M4 internal thread.

[0068] The cage surfaces form a lordosis angle @1 of 3° in the front portion—rear portion direction, and a lordosis angle @2 of 2° in the direction of the centers for the curvature radii of the curved sidewalls 1 and 2. Thus the height of the partition with raised portion is 8.1 mm on the outer side and 7.8 mm on the inner side. The sidewall 1 has two lateral openings which are situated about in the middle of the cavities 7 and 9. The inner radius of curvature R1 is 11 mm, and the outer radius of curvature R2 is 19 mm, the respective centers being 1.1 mm apart. The cage was made of CFK/PEEK, a BYJET water jet cutting apparatus (Bystronic Laser AG, CH-3362 Niederönz) being used in the first process step.

[0069] As a further embodiment example, the cage structure according to FIG. 6 was manufac-tured from a titanium alloy, Ti6-Al4-V according to ISO Standard 5832-3, a BYJET water jet cutting apparatus likewise being used in the first process step. 

1. Cage for a cage-like intervertebral implant, comprising a member divided into cavities, wherein the cage (100) is constituted by a concave-curved sidewall (1), a convex-curved sidewall (2), a front portion (3), and a rear portion (4), a first, inner radius of curvature (R1) being allocated to the inner sidewall (1), and a second, outer radius of curvature (R2) being allocated to the outer sidewall (2); the sidewalls are connected to the front portion and the rear portion; the sidewalls (1, 2) are connected to at least one partition (5, 6) substantially perpendicular to the sidewalls, whereby the cage (100) has at least two cavities (7, 8, 9); and the sidewalls (1, 2), front portion (3), rear portion (4), and the at least one partition (5, 6) form an upper and a lower cage surface (10, 11), the latter forming a lordosis angle (@ 1) in the front portion-rear portion direction, and a second lordosis angle (@2) perpendicular thereto, in that the cage surfaces (10, 11) intersect outside the cage.
 2. Cage according to claim 1, wherein the inner sidewall (1) has a smaller height than the outer sidewall (2), or the outer sidewall (2) has a smaller height than the inner sidewall (1), whereby a second lordosis angle (@2) is formed.
 3. Cage according to claim 1 or 2, wherein the front portion (3) and the rear portion (4) have a substantially equal second lordosis angle (@2).
 4. Cage according to one of claims 1-3, wherein the lordosis angle (@1) is 2-8°, preferably 3, 5, or 7°.
 5. Cage according to one of claims 1-4, wherein the lordosis angle (@2) is 0.1-4°, preferably 2°, and in the limiting case is identically zero.
 6. Cage according to one of claims 1-5, wherein the first, inner radius of curvature (R1) and the second, outer radius of curvature (R2) have substantially the same centers, but preferably centers arranged offset.
 7. Cage according to one of claims 1-6, wherein the inner radius of curva-ture (R1) is 15-23 mm, preferably 22 mm, and the outer radius of curvature (R2) is 18-26 mm, preferably 19, 21 or 23 mm.
 8. Cage according to one of claims 1-7, wherein the inner sidewall (1) has at least one opening (13, 14, 15).
 9. Cage according to one of claims 1-8, wherein the outer sidewall (2) has at least one opening.
 10. Cage according to one of claims 1-9, wherein the front portion (3) is rounded and has at least one beveled surface (23).
 11. Cage according to one of claims 1-10, wherein the rear portion (4) has in its middle a hole (20) with an internal thread, which is surrounded by two guide elements (21, 22) which run substantially parallel or in a V-shape.
 12. Cage according to claim 11, wherein the guide elements [21, 22] are arranged about perpendicular, or about parallel, or at an angle of 0° to 90°, preferably 45°, to the cage surfaces around the hole (20).
 13. Cage according to claim 11 or 12, wherein the guide elements (21, 22) are constituted as ribs or recesses.
 14. Cage according to one of claims 1-13, wherein holes (31, 32, 33) are provided in the front portion (3) and in the rear portion (4) for markers of a metal of high density as balls and/or pins.
 15. Cage according to claim 14, wherein tantalum is provided as the metal for the markers.
 16. Cage according to claim 14 or 15, wherein the markers are arranged as pins about perpendicular or parallel to the hole (20).
 17. Cage according to one of claims 1-16, wherein the cage surfaces (10, 11) have tabular raised portions (24, 25, 26, 27) which run substantially parallel to the cage surfaces.
 18. Cage according to one of claims 1-17, wherein it has a tilting moment (M) directed in the direction of the center of the inner radius of curvature (R1) and, in comparison with the tilting moment of a cuboidal cage of the same cross section, same length and comparable cage structure, exceeding this tilting moment by a factor of at least 1.30.
 19. Cage according to one of claims 1-18, wherein it consists of polyether ether ketone (PEEK), polyether ketone ether ether ketone (PEKEEK), or of polysulfone (PS), or of a composite material, preferably carbon fiber reinforced composites of CFK/PEEK and CFK/PEKEEK.
 20. Cage according to one of claims 1-18, wherein it consists of titanium or a Ti alloy.
 21. Cage according to claim 20, wherein it has a hydroxyapatite ceramic (HAK) coating or a tricalcium phosphate (TCP) coating.
 22. Cage according to one of claims 1-21, wherein it has a Cage Mass Index (CMI) according to Equation (1) CMI=Volume of cage material/Volume of cage  (1) which for CFK/PEEK, CFK/PEKEEK, and PS is less than 0.25, preferably 0.22, and for titanium or Ti alloys, less than 0.20, preferably 0.17.
 23. Process for the production of a cage according to one of claims 1-22, wherein in a first step a blank of cage material is machined in a first direction by means of an ultrahigh pressure water jet, the contours of the sidewalls (1, 2), of the front portion (3), the rear portion (4), the at least one sidewall (5, 6), the at least two cavities (7, 8, 9), and the guide elements (21, 22) arising; and in a second step the blank is milled in a direction substantially perpendicular to the first direction; and in a third step, the markers are installed; and finishing follows in a fourth step.
 24. Process according to claim 23, wherein, in the second step, the cage surfaces (10, 11) are milled according to the first lordosis angle (@1), the tabular raised portions (24, 25, 26, 27) are milled corresponding to the second lordosis angle (@2); and the beveled surfaces (23, 23@) of the front portion (3), the at least one opening (13, 14, 15) of the sidewall (1), and the first hole (20) are milled.
 25. Process according to claim 24, wherein the first hole (20) is provided with an internal thread.
 26. Process according to one of claims 22-25, wherein, in the third step, second holes (31, 32, 33) are formed for the markers, and the markers are inserted.
 27. Process according to one of claims 22-26, wherein the final processing of the machine blank includes trovalization, marking, cleaning, and packaging. 